Measuring points of this type include, for example, pH measuring points or other measuring points of analytical measurements technology. In this category belong, especially, other potentiometric, amperometric, coulometric, colorimetric, photometric, turbidimetric and spectrometric units.
The problem to which the invention is directed will be explained on the basis of an example of pH measuring points. It is not intended, however, that the invention be limited to a method for operating pH measuring points.
A sensor unit includes at least one transducer, which outputs an electrical signal dependent on the value of the measured variable. Frequently, the transmit function of the transducer is variable. This is true in special measure for pH sensors. Therefore, pH sensors, or pH electrodes, must, at suitable points in time, be subjected to maintenance, especially, it must be calibrated anew. The length of time intervals between the maintenance procedures, or calibration intervals, depends strongly on the environmental influences, to which the sensor is exposed during its lifetime. Also, the total lifetime of the sensor is strongly influenced by these environmental conditions.
The term, “calibrating”, has frequently a somewhat different meaning in pH measuring than usual. In general, one means with “calibrating” the checking of the display of a measuring device against a standard; the deviation between true value and display value is detected. The conforming of the display value to the true value is referred to as adjusting. “Calibrating” in the case of a pH sensor is strictly an adjusting. Since the term, “calibrating”, is widely used in electrochemistry, it is also used here.
In the guidelines VDI/VDE 2650 and NAMUR NE107, recently, future market developments in the area of sensor diagnostics are emphasized.
The state of the art contains attempts to equip sensors with intelligent self-diagnosis. Thus, there are first publications with reference to analysis of individual sensor characteristics for ascertaining the period of time until the entering of a defined sensor state in the future, e.g. the end of the sensor lifetime or a point in time, at which a new calibrating is necessary. Cited by way of example are DE 10141408, JP 05-209858, JP 2002-228495, DE 10 2004 012420 and DE 10239610.
The described methods in the named publications operate on the assumption that the behavior of a sensor in a medium with essentially known ingredients and in the case of known environmental conditions can be sufficiently well described by a model. Actually, however, the relationships are a number of times more complex, so that a mapping via a model of the processes, which influence the transmit function of a sensor, especially, a pH electrode, is not, or, at most, only with large effort, possible.
pH glass electrodes embodied as single-rod, measuring chains are subject to wear, which depends, in large part, on pH value and temperature of the environment. Besides the glass membrane, also the reference half-cell of a glass electrode is strongly loaded. While, at the glass membrane, due to media influences, a gradual deterioration takes place and thereby the probability for a sensor failure increases, diaphragm changes, poisoning and deterioration of the reference electrolyte of the reference half-cell can also degrade the functionality of the sensor.
Due to the large number of parameters, which determine the lifetime, or service life, of a sensor, prediction is difficult, the more so, since also a considerable scatter from sensor instance to sensor instance of the same type occurs.